This invention relates to an exhaust valve and, in particular, to a slow release valve for use with an automatic sphygmomanometer which measures human blood pressure.
An automatic sphygmomanometer which uses an exhaust valve is shown in FIG. 1. A cuff and a pressurizing pump for feeding air into the cuff, indicated by reference numerals 21 and 22, respectively, are provided, between which an exhaust valve 1 for slow exhaust and another exhaust valve 23 for rapid exhaust are interposed. Valves 1 and 23 are fluidically connected with each other as well as with the cuff and the pump, the cuff 21 being also connected to a pressure detecting means 24 composed of a bellows and a differential transformer. Reference numeral 25 indicates a microphone for sound detection, 26 an amplifier, 27 a low-pass filter, 28 a band-pass filter, 29 a level detecting circuit, 30 a Korotkoff's sound discriminator, 31 an operation control circuit, 32 a converter, 33 a pulse frequency computation circuit, 34 a memory circuit for storing maximum and minimum blood pressure and pulse frequency values, and 35 a display for displaying these values.
During operation of the FIG. 1 apparatus, air pressure in the cuff is increased to exceed the maximum blood pressure value, and then is gradually reduced by bleeding air through the slow release exhaust valve 1. After maximum and minimum blood pressure measurements and a pulse frequency measurement are taken, the air in the cuff 21 is rapidly exhausted through the rapid exhaust valve 23.
It is generally known that when measuring blood pressure, discrimination of Korotkoff's sounds using an automatic sphygmomanometer according to the Riva-Rocci-Korotkoff method is most easily done when the slow release exhaust rate, e.g. the release rate through exhaust valve 1 in FIG. 1, is about 2 to 3 mmHg/sec. However, it is impossible to maintain an optimum exhaust rate under a wide range of pressure conditions because conventional slow release exhaust valves in general have a device such as a plate in which a small exhaust hole is bored which causes a rapid exhaust when the air pressure is high, but a slower exhaust when the air pressure is low, as shown in FIG. 5.